(1) Field of the Invention
The present invention relates to a pneumatic tire for a two-wheeled vehicle, which is improved in its motion characteristics on running at high speeds by suitably setting rigidity of the tire in the circumferential direction at the central portion and side edge portions of the tread of the tire.
(2) Related Art Statement
With the recent development of expressway networks and improvement of vehicle performance, various pneumatic radial tires for two-wheeled vehicles have been proposed. The pneumatic radial tire of this kind includes a carcass composed of at least one carcass ply of organic fiber cords arranged at a constant angle of 75.degree. to 90.degree. relative to the tire equatorial plane, and a belt arranged between the carcass and the tread rubber and composed of at least one belt layer of cords uniformly arranged at small angles relative to the tire equatorial plane. With such a tire, the belt tightens the carcass in the circumferential direction to restrain the movement of the tread. Therefore, it is known that the tire of this kind produces less heat, and exhibits high wear-resistance, high gripping performance, and excellent performances at high speeds.
Moreover, another pneumatic radial tire for a two-wheeled vehicle has been recently proposed, wherein a belt is composed of a belt layer of organic fiber cords spirally wound substantially parallel to the tire equatorial plane. By so doing, the bending rigidity of the tread is increased in the tire circumferential direction to restrain radially outward swelling of the tread and to improve the straight running performance at high speeds with the tire weight being reduced. As the organic fiber cords, aromatic polyamide fibers or the like having a high modulus of elasticity of not less than 600 kg/mm.sup.2 are generally used in consideration of the radially outward swelling of the tread, particularly at its central portion.
In the tire having the organic fiber cords spirally wound and having the high modulus of elasticity, however, the bending rigidity of the tread in the circumferential direction is increased at its central portion owing to the arrangement of the organic fiber cords so that the bending rigidity of the tread at the side edge portions including sidewalls is also increased. On the other hand, the tire for the two-wheeled vehicle is inclined relative to a road surface when the vehicle is turned at a corner. Therefore, the tire for the two-wheeled vehicle is susceptible to external disturbances due to projections and depressions and differences in level on the road, so that running stability is deteriorated. This problem becomes particularly conspicuous in the case of tires having aspect ratios decreased for high performance tendency. Moreover, the organic fiber cords having high moduli of elasticity, such as aromatic polyamide fiber cords, are more expensive in comparison with the organic fiber cords having low moduli of elasticity, such as nylon 6 or nylon 66. Accordingly, the tire of the prior art involves an economical problem yet to be solved.